The present invention relates generally to an improved process for the production of high purity meta-xylene as part of an integrated xylene separation and recovery system utilizing an improved arrangement of fractionation, adsorption and crystallization steps.
Commonly, para-xylene is commercially produced from C-8+ aromatic streams, usually the C-8+ fraction of reformate from catalytic reformers. The C-8+ aromatics are typically fed to a distillation column, commonly referred to as a xylene splitter, where the ethylbenzene, para-xylene, meta-xylene, and some or virtually all of the ortho-xylene are taken overhead. Ortho-xylene is normally left in the xylene splitter bottoms stream in a significant amount only if a purified ortho-xylene product is to be produced. If it is, then the xylene splitter bottoms stream is typically fed to an ortho-xylene product column where the ortho-xylene is distilled from the C-9+ aromatics.
The xylene splitter overhead stream is then fed to a para-xylene purification (PXP) unit for the recovery and purification of para-xylene product. This PXP unit separates out the para-xylene, typically either by crystallization or by adsorption onto a molecular sieve, utilizing conventional technologies. The para-xylene-depleted stream withdrawn from the PXP unit is then fed normally to an isomerization unit where the meta-xylene and ortho-xylene are partially converted catalytically to para-xylene. Depending on the type of catalyst, the ethylbenzene in this stream is partially converted in the isomerization unit primarily to either benzene or xylenes. After distilling off toluene and lighter compounds, the C-8+ aromatics from the isomerization unit are fed back to the xylene splitter.
If it is desired that meta-xylene be separated out as a purified product in this type of plant, U.S. Pat. No. 3,773,846 (Berger ""846), which is incorporated herein by reference, and other patents (described below) have proposed that meta-xylene purification be done between the PXP unit and the isomerization unit. As with para-xylene, meta-xylene can be purified by either crystallization or adsorption. If crystallization is used, however, the purification of the meta-xylene can be hindered by the para/meta and ortho/meta eutectics. Thus, U.S. Pat. Nos. 3,798,282 (Bemis et al.) and U.S. Pat. No. 3,825,614 (Bemis et al.), which are incorporated herein by reference, teach methods for purifying meta-xylene downstream of a para-xylene crystallization unit by crystallization at temperatures below the para/meta eutectic. These patents teach crystallization techniques whereby the para-xylene crystals will be smaller than the meta-xylene crystals, and this allows for some degree of rough separation. After this first separation, the concentrated meta-xylene can be melted and recrystallized in a second stage to produce high-purity meta-xylene. Using adsorption to purify the para-xylene, as taught Berger ""846 is probably a more practical commercial approach when meta-xylene is to also subsequently be purified by crystallization because adsorption can reduce the para-xylene concentration in the meta-xylene crystallization unit feed to well below the para/meta eutectic point and, thus, avoid this eutectic problem.
Berger ""846 does not, however, efficiently address the issue of avoiding the meta/ortho eutectic which can form during meta-xylene purification by crystallization. In Berger ""846, the ortho-xylene removal is accomplished by fractionation, in particular in a xe2x80x9cthird fractionation zonexe2x80x9d, located between the adsorption step and the meta-xylene purification step. Also in Berger ""846, a xe2x80x9csecond fractionation zonexe2x80x9d is recommended to be located just upstream of the third fractionation zone. This second fractionation zone is used to concentrate the meta-xylene prior to crystallization by distilling away much of the ethylbenzene. This approach is expensive in terms of both capital and operating costs.
A variety of other approaches to the purification of meta-xylene from mixtures of C-8 aromatic hydrocarbons are known in the art. For example, one familiar approach is a liquid/liquid extraction process as taught by U.S. Pat. Nos. 2,528,892; 2,738,372; 2,848,517; 2,848,518; 3,309,414; 3,515,768; and 3,584,068. The liquid/liquid extraction process of these patents is based on the knowledge that boron trifluoride (BF3) and hydrofluoric acid (HF) will form a complex with meta-xylene that is more stable than the corresponding complexes with para- or ortho-xylenes. When mixed xylenes are mixed with limited amounts of BF3 and large amounts of HF, the xylene/BF3/HF complexes form a heavy, acid, liquid phase that separates from a lighter, hydrocarbon, liquid phase. The acid phase has a high percentage of meta-xylene with lesser amounts of para- and ortho-xylenes. Non-aromatic hydrocarbons do not form complexes and thus stay in the hydrocarbon phase. Depending on conditions, ethylbenzene either stays with the light phase or disproportionates to benzene and diethylbenzene, which can also complex with BF3/HF. Through the use of counter-current contacting, the heavy complex can be stripped of nearly all of the non-meta-xylene hydrocarbons and most of the meta-xylene can be extracted from the light hydrocarbon phase. The complex can then be broken, and the meta-xylene separated out. The ortho-xylene and ethylbenzene can be separated from the para-xylene and residual meta-xylene by distillation.
One of the principal disadvantages of this type of liquid/liquid extraction process is that boron trifluoride and hydrofluoric acid are both extremely corrosive and dangerous chemicals. The environmental and safety risks associated with this type of process are so high as to be unacceptable to many companies and countries today. Also, this type of plant is expensive because of the many pieces of equipment and because of the cost of metals needed to withstand the corrosive nature of the chemicals.
Other known exaction-type purification processes use different chemicals but suffer from similar drawbacks. Thus, U.S. Pat. No. 2,830,105 teaches an extraction with phosphorus pentafluoride and hydrofluoric acid; U.S. Pat. No. 3,707,577 uses lithium chloride and aluminum chloride as part of a somewhat more complex extraction process; and, U.S. Pat. No. 2,562,068 employs a double solvent extraction using sulfur dioxide and pentane.
It is also known to purify meta-xylene by selective reaction followed by some type of separation. In U.S. Pat. Nos. 2,889,382 and 3,644,552, meta-xylene is selectively halogenated in the presence of para-xylene. Subsequent processing by distillation, crystallization and/or adsorption can be used to separate the halogenated meta-xylene from other C-8 aromatic hydrocarbons. The halogenation process, however, is not entirely selective for meta-xylene, and the patented processes employ expensive catalysts and/or highly corrosive materials such as molecular chlorine, hydrochloric and nitric acids. U.S. Pat. No. 2,511,711 teaches selectively sulfonating meta-xylene. As with selective halogenation, this process is not entirely selective for meta-xylene and utilizes highly corrosive chemicals. Still a third variety of selective reaction is selective alkylation of meta-xylene with propylene as taught by U.S. Pat. No. 3,539,650. This process also has many disadvantages. First, the alkylation conversion is very low which results in an expensive alkylation unit. Second, there are reaction yield losses not just once but twice. Third, propylene is needed. Fourth, cumene is made, and usually making additional products is not desirable. Fifth, the processing is very complicated and expensive.
Yet another conventional approach to purification of meta-xylene is an extractive distillation process. U.S. Pat. No. 2,763,604 teaches the use of benzonitrile and similar compounds and mixtures as an extractive distillation solvent. Benzonitrile forms loose complexes with aromatics with the strongest effect being with meta-xylene. The addition of the benzonitrile makes the meta/para separation practical with distillation. It also appears that the ethylbenzene from meta/para separation is easier. Because benzonitrile has a significant vapor pressure, however, the products from this type of distillation would probably need to be water washed and the benzonitrile recovered from the wash. In general, processes using solvents that must be made up are not favored because of the added cost and because of concerns about product contamination. Similarly, U.S. Pat. No. 3,089,829 teaches extractive distillation with benzoic acid, and U.S. Pat. No. 3,849,261 teaches extractive distillation with organo-metallic compounds.
Another well-known approach to the purification of meta-xylene is to use crystallization technology without related adsorption treatment. One such process is described in U.S. Pat. No. 2,884,470 which employs three crystallizers in series. The first is an equilibrium crystallizer that crystallizes para-xylene down to a point just above the para/meta eutectic. The second crystallizer is fed the mother liquor from the first crystallizer. This second crystallizer is xe2x80x9csupercooledxe2x80x9d in that para-xylene crystals form but meta-xylene crystals do not form because meta-xylene requires more subcooling to initiate crystal formation. Thus, the mother liquor from this second crystallizer is on the meta-xylene side of the meta/para eutectic composition. The third crystallizer crystallizes meta-xylene from the mother liquor from the second one, which is made possible because the crystallizer is seeded with meta-xylene crystals. A similar process is described in U.S. Pat. No. 2,777,888, where first para-xylene is crystallized down to a temperature below the meta/para eutectic, then ortho-xylene is crystallized with the aid of seeding, and then meta-xylene is crystallized with the aid of seeding. Such crystallization processes, which rely on preferential crystallization to avoid eutectic problems, have not been commercialized, perhaps because impurities and/or equipment surfaces in a commercial installation trigger premature crystallization of some system components.
U.S. Pat. No. 3,277,200 teaches a process for co-crystallizing meta-xylene and para-xylene followed by selectively melting the para-xylene crystals to separate them from the meta-xylene. This patent, like several others that are discussed below, takes advantage of the fact that when meta-xylene is crystallized with lesser amounts of para-xylene (i.e., starting with a meta/para mixture with an initial crystallization temperature just above the eutectic point), the para-xylene crystals tend to be smaller. In all of these patents, the para-xylene is recovered by crystallization within the limits of the meta/para eutectic, and then the mother liquor is further crystallized to form both meta and para crystals. In this patent, the resulting slurry is then heated in a controlled way, which preferentially will first melt the smaller crystals, and then before thermal equilibrium is reached, the slurry is quickly filtered. The resulting cake reportedly can be about 95 percent meta-xylene. A second crystallization, which is not discussed in any detail, could achieve a higher meta-xylene purity. This patent is specifically directed to the use of feeds to the meta-xylene crystallization step containing no more than 3 percent of ethylbenzene and no more than 3 percent ortho-xylene. There is no discussion about what happens if ortho-xylene concentration in the feed is high enough to also crystallize ortho-xylene in the crystallization step. By contrast, the present invention avoids the second meta-xylene crystallization, is not limited to low ethylbenzene concentrations, and addresses the issue of ortho-xylene crystallization.
U.S. Pat. No. 3,544,646 teaches another process for co-crystallizing meta-xylene and para-xylene followed by a separation between meta- and para-xylene based on crystal density. According to this patent, the density of para-xylene crystals is 1.006 g/cc, and the density of meta-xylene crystals is 1.030 g/cc. Para-xylene is first recovered by crystallization close to the eutectic point. Then the mother liquor is further crystallized with enough miscible liquid diluent (e.g., freon) so that the liquid phase in the slurry leaving the second crystallizer has a density between the densities of the two types of crystals. By some sort of separator, presumably a bowl-type centrifuge, the two types of crystals are separated. Thus, a meta-rich stream and a para-rich stream are produced, and each can be recrystallized to make high purity products. The crystal density differences are so small, however, that this idea seems impractical in a commercial operation. This approach also requires large amounts of liquid freon that must also be cooled and recovered, and the diluent lowers the temperatures needed for the same meta-xylene recovery.
A patent somewhat similar in concept to the above-mentioned ""646 patent is U.S. Pat. No. 3,798,282 which teaches co-crystallizing meta-xylene and para-xylene followed by a separation based on differences in crystal settling rates. This patent teaches that because the meta-xylene crystals produced are relatively large, they will settle faster in the slurry compared with the smaller para-xylene crystals thereby permitting separation. The ""282 patent also suggests advantages to removing ortho-xylene from the feed as a bottoms stream from a xylene splitter before carrying out the meta-/para-xylene crystallization on the remaining feed. In the ""282 patent, the ortho-xylene is sent to an isomerization step as an overhead stream from an ortho-xylene column, comparable to the treatment of ortho-xylene in some embodiments of the present invention.
The present invention differs from the ""282 patent, however, in several critical respects. First, the present invention uses adsorption rather than co-crystallization/crystal settling for para-xylene separation. Second, partially because the adsorption step alters the para/meta eutectic, crystallization units in the present invention are designed differently and are more efficient. Third, the present invention proposes several ways to separate out the ortho-xylene that are different from what is taught by the ""282 patent. Fourth, the concept of using a disproportionation unit instead of the isomerization unit to isomerize recycled ortho-xylene is not taught by the ""282 patent.
U.S. Pat. No. 3,825,614 describes still another variation on the co-crystallization concept in which meta-xylene and para-xylene are co-crystallized but under operating conditions such that the para-xylene crystallization rate is substantially lower than the meta-xylene crystallization rate. This process is dependent on starting with a relatively low para-xylene concentration of less than 10 percent, preferably less than 8 percent.
The ""614 patent describes two ways to get the para-xylene concentration sufficiently low. First, a diluent, such as toluene, butane, naphthenes, carbon dioxide or ethane can be used. Second, some of the eutectic mixture can be crystallized out in a separate crystallizer before doing the nonequilibrium crystallization. The first technique leads to the complications of introducing a third component into these crystallization systems, while the second approach adds another crystallization treatment, which is costly.
Another variety of meta-xylene purification technology utilizes adsorption of para-xylene by itself or in combination with some type of downstream treatment to achieve greater purification of the meta-xylene. U.S. Pat. No. 3,700,744 is representative of the former approach. In this patent, the ethylbenzene is first distilled off and reacted to extinction in a loop through an isomerization unit. Then the ortho-xylene and C9+ aromatics are all fractionated off the bottom of a xylene splitter. Finally, the remaining stream, which is a relatively pure mix of meta- and para-xylene, is fed to a para-xylene adsorption unit where the two xylenes are separated. This approach to meta-xylene purification, however, is impractical and not economically viable for three reasons. First, it requires the fractionation separation of ethylbenzene from para-xylene, which according to the patent requires about 200 trays and a reflux ratio of about 15 resulting in high capital and energy costs. Second, the separation in the xylene splitter is quite difficult because the split between ortho-xylene and meta-xylene must be very complete. Third, if there are nonaromatics in the feed that boil with meta-xylene, then these compounds will end up in the meta-xylene product. Because it will be difficult to keep nearly all nonaromatics, ortho-xylene, and ethylbenzene out of the feed to the adsorption unit, high purity meta-xylene cannot be efficiently produced using this design.
Several patents, such as U.S. Pat. No. 5,382,747, teach processes in which a para-xylene adsorption is combined with downstream processing of the meta-xylene. U.S. Pat. Nos. 3,770,841 and 3,729,523 describe adsorption processes followed by distillation of meta-xylene. In the ""841 patent, two adsorption units are used to recover para-xylene and ethylbenzene as separate pure products. The remaining meta-xylene and ortho-xylene are then purified by distillation. Isomerization is used to convert meta- and/or ortho-xylene to para-xylene. U.S. Pat. No. 3,729,523 is essentially the same as the ""841 patent except that the para-xylene and ethylbenzene come off the initial adsorption together. The para-xylene is then separated from the ethylbenzene by crystallization. The problem with these approaches is that neatly all of the para-xylene and ethylbenzene must be recovered as purified products from the adsorption step. Any unrecovered para-xylene or ethylbenzene will end up the meta-xylene product. This requires that the upstream adsorption units must operate so as to provide nearly 100 percent product purity and nearly 100 percent recovery which is not practical.
More relevant to the present invention is U.S. Pat. No. 3,773,846 (Berger), previously discussed. This patent describes a meta-xylene purification process combining para-xylene adsorption with subsequent fractionation of the raffinate to decrease the concentration of ortho-xylene or of an ortho-xylene/ethylbenzene mix and a subsequent meta-xylene crystallization. In either case, the fractionation must reduce the ortho-xylene concentration low enough so as to avoid the ortho/meta eutectic in the subsequent meta-xylene crystallization unit. The differences between this patent and the present invention have been addressed above.
All of the foregoing prior art processes for purifying a meta-xylene product as part of an integrated xylene separation and purification operation therefore have various disadvantages and drawbacks. High costs are incurred due to equipment, maintenance and operating expenses in these prior art processes. These and other drawbacks with and limitations of the prior art processes are overcome, in whole or in part, with the improved meta-xylene purification process of this invention.
Accordingly, a principal object of this invention is to provide an improved process for purifying meta-xylene from a C-8+ aromatic feed stream.
It is a specific object of this invention to provide an efficient and economical approach to preparing high-purity meta-xylene as part of an integrated xylene separation operation.
It is also an object of this invention to provide a process for purifying meta-xylene from a C-8+ aromatic feed which reduces capital and energy costs by minimizing the number of times that ethylbenzene, ortho-xylene and meta-xylene are distilled.
Still another object of this invention is to provide an improved integrated continuous process for treating a C-8+ aromatic feed for separation and recovery of the para-, meta-, and ortho-xylene components thereof utilizing novel, integrated arrangements of xylene distillation, adsorption, crystallization, and isomerization treatment steps.
Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the processes and related apparatus, involving the several steps and the various components, and the relation and order of one or more such steps and components with respect to each of the others, as exemplified by the following description and the accompanying drawings. Various modifications of and variations on the process and apparatus as herein described will be apparent to those skilled in the art, and all such modifications and variations are considered within the scope of the invention.
In general, this invention comprises a multi-step continuous process for treating a C-8+ aromatic feed stream. As used herein, the term xe2x80x9ccontinuous processxe2x80x9d is meant to refer to an overall process wherein there are continuous feeds into and out of the system, including processes involving intermediate semi-batch processing of certain streams within the system. An aromatic feed is fed to a xylene splitter column, which is operated so as to produce an overhead stream typically comprising predominantly para- and meta-xylenes, with lesser amounts of ethylbenzene and ortho-xylene. A significant amount of the ortho-xylene fed to the xylene splitter is taken off in the bottoms stream with nearly all of the C-9+ aromatic compounds and/or is taken off in a side stream from the column. The amount of ortho-xylene allowed to go overhead in the xylene splitter is kept low enough to avoid the crystallization of ortho-xylene in the downstream meta-xylene crystallization unit. The xylene splitter overhead stream is sent to an adsorption step that separates out high-purity para-xylene and leaves a residual stream, which typically would be over 50 percent meta-xylene. This meta-xylene-rich stream is then sent to a crystallization step that separates out high-purity meta-xylene and leaves a mother liquor stream which goes to the isomerization unit. The recovery of para-xylene in the adsorption unit is sufficiently high so as to avoid crystallizing para-xylene in the meta-xylene crystallization step.
Except for ortho-xylene in the xylene splitter bottoms that is further fractionated to produce a purified ortho-xylene product, the rest of the ortho-xylene either in the bottoms or in a side stream is sent to an isomerization step. This xe2x80x9cisomerization stepxe2x80x9d can be in the isomerization unit, which was fed the crystallization unit mother liquor, or a disproportionation unit which also produces equilibrium xylenes. In general, the isomerization step(s) will be operated (by choice of catalyst, temperature, etc.) so as to approach thermodynamic equilibrium among the xylenes. The C-8+ aromatics from each isomerization step are passed back to the xylene splitter for recycle through the process.
Taken individually, some of the component steps of this invention are generally well-known for one or another use in the art of hydrocarbon processing. No previous xylene separation and purification process, however, is believed to have combined and operated these processing steps in the particular ways described in this invention, and no prior art in this field suggests the surprisingly efficient and advantageous results obtained thereby.